A modulation scheme used for wireless communication devices such as a modern mobile phone and the like has radio-frequency utilization efficiency and a high peak-to-average power ratio (PAPR). In order to amplify a signal to which an amplitude modulation is applied by using an AB class amplifier that has been commonly used in a wireless communication field, it is necessary to use an amplifier operating with sufficient back-off to maintain a linearity. Generally, the required back-off value is at least approximately equal to a value of the PAPR. However, in the AB class amplifier, the maximum efficiency is obtained when it operates at the saturation point and the efficiency of the amplifier decreases with increasing the back-off value. Therefore, it is difficult to improve the power efficiency of the power amplifier for amplifying the radio-frequency modulation signal having a high PAPR.
As a power amplifier for amplifying a modulation signal having a high PAPR with high efficiency, a polar modulation power amplifier is used. In the polar modulation power amplifier, the radio-frequency modulation signal used for wireless communication is generated from the polar coordinate components of amplitude and phase. FIG. 15 is a configuration diagram of a polar modulation power amplifier described in non-patent document 1. The amplifier comprises a radio-frequency modulation signal input terminal 101, an amplitude signal input terminal 102, a power supply modulator 103, a radio-frequency power amplifier 104, and a radio-frequency modulation signal output terminal 105. The power supply modulator 103 comprises a linear amplifier 106, a subtractor 107, a current sensing resistor 108, a hysteresis comparator 109, a switching amplifier 110, an inductor 111, and an electric power supply terminal 112.
The radio-frequency modulation signal that is amplitude-modulated or phase-modulated is inputted through the radio-frequency modulation signal input terminal 101 and outputted to the radio-frequency power amplifier 104. An amplitude signal in the radio-frequency modulation signal inputted through the radio-frequency modulation signal input terminal 101 is inputted from the amplitude signal modulation terminal 102. The signal inputted through the amplitude signal modulation terminal 102 is highly efficiently amplified by the power supply modulator 103 and supplied to the radio-frequency power amplifier 104 through the electric power supply terminal 112 as a power supply. The radio-frequency power amplifier 104 amplifies the signal inputted through the radio-frequency modulation signal input terminal 101 and outputs the amplified signal to the radio-frequency modulation signal output terminal 105.
To amplify the input signal with high efficiency and low distortion, the power supply modulator 103 has a configuration in which both the switching amplifier 106 and the operational amplifier 110 are disposed. The amplitude signal inputted through the amplitude signal modulation terminal 102 is inputted to the linear amplifier 106.
The output impedance of the linear amplifier 106 is low. The linear amplifier 106 linearly amplifies the inputted signal and outputs the amplified signal. The signal outputted by the linear amplifier 106 is transmitted to the electric power supply terminal 112 through the current sensing resistor 108.
The subtractor 107 is connected to both ends of the current sensing resistor 108 and outputs a value obtained by subtracting a voltage of the electric power supply terminal 112 from a voltage of the output signal of the linear amplifier 106. Here, because the input impedance of the subtractor 107 is high, the subtractor 107 does not consume a large amount of electric power supplied to the output signal of the linear amplifier 106 and the electric power supply terminal 112.
Further, because the impedance of the current sensing resistor 108 is set to low, the voltage across both ends of the current sensing resistor 108 is negligibly small compared to the voltage on the electric power supply terminal 112.
The output signal of the subtractor 107 is inputted to the hysteresis comparator 109. The hysteresis comparator 109 makes a sign determination of the input signal and outputs a result to the switching amplifier 110. However, the hysteresis comparator 109 has a function to hold the latest output state and has a hysteresis width (V_hys), when the latest output state is “Low”, the output state changes to “High” when the input signal level becomes equal to or greater than V_hys/2 and when the latest output state is “High”, the output state changes to “Low” when the input signal level becomes equal to or smaller than −V_hys/2.
The signal inputted to the switching amplifier 110 is amplified and outputted to the electric power supply terminal 112 through the inductor 111. In this operation, a current supplied by the switching amplifier 110 through the inductor 111 is combined with a current supplied by the linear amplifier 106 through the current sensing resistor 108, and the current is sent to the electric power supply terminal 112.
The above-mentioned power supply modulator 103 has two advantages: high linearity of the linear amplifier 106 and high efficiency of the switching amplifier 110. This is because in the power supply modulator 103, the output voltage is determined by the linear amplifier 106 having low output impedance and most of the output current is supplied by the switching amplifier 110 with high efficiency. The current outputted through the electric power supply terminal 112 is a sum of the output current of the linear amplifier 106 and the output current of the switching amplifier 110. An electric potential of the electric power supply terminal 112 depends on the electric potential of the linear amplifier 106 having low output impedance. In order to maintain the electric potential of the electric power supply terminal 112 to a target value, the current is supplied by the linear amplifier 106. The output current of the linear amplifier 106 is detected by using the current sensing resistor 108 and the hysteresis comparator 109 and the current supplied by the switching amplifier 110 is adjusted so that the output current of the linear amplifier 106 is prevented from becoming excessive. By using the above-mentioned method, most of the current outputted through the electric power supply terminal 112 is supplied by the switching amplifier 110 and the output current of the linear amplifier 106 can be used only for correction of an error component of the switching amplifier 110.
Additionally, as with non-patent document 1 shown in FIG. 15, an example using the power supply modulator in which the switching amplifier and the linear amplifier are operated together is described in patent document 1.
FIG. 16 is a configuration diagram of a radio-frequency power amplifier described in patent document 1. The radio-frequency power amplifier comprises a radio-frequency modulation signal input terminal 201, an envelope detector 202, a limiter 203, a power supply modulator 204, a radio-frequency power amplifier 205, and a radio-frequency modulation signal output terminal 206. Further, the power supply modulator 204 comprises a delta modulator 207, a switching amplifier 208, a low pass filter 209, an operational amplifier 210, an attenuator 211, an adder 212, and an electric power supply terminal 213.
The radio-frequency modulation signal that is amplitude-modulated or phase-modulated is inputted through the radio-frequency modulation signal input terminal 201 and sent to the envelope detector 202 and the limiter 203. The envelope detector 202 extracts only the amplitude modulation component from the inputted radio-frequency modulation signal and outputs the extracted amplitude modulation component to the power supply modulator 204. The power supply modulator 204 amplifies the signal inputted by the envelope detector 202 with high efficiency and supplies the amplified signal through the electric power supply terminal 213 as a power supply for the radio-frequency power amplifier 205. The limiter 203 eliminates the amplitude modulation component from the signal inputted through the radio-frequency modulation signal input terminal 201 and outputs the limited signal to the radio-frequency power amplifier 205. The radio-frequency power amplifier 205 amplifies a signal obtained by multiplying the signal inputted by the limiter 203 by the signal supplied through the electric power supply terminal 213, and outputs the amplified signal through the radio-frequency modulation signal output terminal 206.
To amplify the input signal with high efficiency and low distortion, the power supply modulator 204 has a configuration in which both the switching amplifier 208 and the operational amplifier 210 are disposed like the power supply modulator 103 shown in FIG. 15. The signal outputted by the envelope detector 202 is inputted to the delta modulator 207 and the operational amplifier 210. The delta modulator 207 creates a 1-bit signal based on the signal inputted by the envelope detector 202 and the feedback signal from the output of the switching amplifier 208, and outputs the 1-bit signal to the switching amplifier 208. The switching amplifier 208 amplifies the signal inputted by the delta modulator 207, outputs the amplified signal to the low pass filter 209, and also returns the amplified signal to the delta modulator 207 as the feedback signal. The low pass filter 209 eliminates high-frequency noise from the signal inputted by the switching amplifier 208 and outputs the filtered signal to the adder 212. On the other hand, the operational amplifier 210 amplifies a signal obtained by subtracting the signal inputted through the attenuator 211 from the signal inputted by the envelope detector 202 and outputs the amplified signal to the adder 212.
The adder 212 adds the output signal of the low pass filter 209 to the output signal of the operational amplifier 210 and outputs the added signal to the attenuator 211 and the electric power supply terminal 213. The attenuator 211 attenuates the output signal of the adder 212 and outputs the attenuated signal to the operational amplifier 210.
In the power supply modulator 204 shown in FIG. 16, the operational amplifier 210 corrects the error component of the switching amplifier 208 like the power supply modulator 103 shown in FIG. 15. Therefore, the power consumption of the operational amplifier 210 is low and most of the electric power supplied to the radio-frequency power amplifier 205 through the electric power supply terminal 213 is supplied by the switching amplifier 208.
Further, the power supply modulator 103 disclosed in non-patent document 1 and the power supply modulator 204 described in patent document 1 can be used as a stand-alone power amplifier. For example, it can be used as a power amplifier for driving a motor.